This study is an application of Freud’s psychoanalysis to Toni Morrison’s Beloved. It is aimed at describing the major cause of Sethe’s doing Mercy Killing, the way of Mercy Killing and the impacts after doing Mercy Killing. The findings of the study show that Sethe’s Mercy Killing is caused by her past experience when she was a slave. The problems result from the disharmony of the id, ego and superego in Sethe. This study reveals that finally Sethe’s superego controls her personality which leads her to kill her baby. Sethe’s mercy killing happens because of some causes which lead her to do mercy killing.